Trauma is the leading cause of death in the United States for men and women under the age of 45 years and the fourth overall cause of death for all ages. Trauma also has a substantial economic impact on the health care system, accounting for over one-third of all emergency department visits and resulting in over $80 billion per year in direct medical care cost, for example, in 2007, over 180 000 people died of trauma, and abdominal injuries contributed to a large number of these deaths.
Pneumoperitoneum is a condition in which a free gas pocket or tiny amount of free gas or air is trapped within the abdominal cavity but not contained in a hollow viscus. Identifying abnormal intra-abdominal gas pockets or collections may be critically important in establishing an accurate diagnosis. Increasing evidence supports that ultrasound imaging is a very useful tool for diagnosis of pneumoperitoneum with abnormal air/gas patterns because of its high accuracy and superiority as compared to plain X-ray radiography. The sonographic air can be outlined as comprising two categories: physiological air or normal air; and pathologic air or abnormal air. Physiological air is air in the gastrointestinal tract and lungs (air projecting into the abdominal cavity).
Bedside ultrasound or Point-of-care Ultrasound is widely used in emergency medicine for initial screening and enables selection of hemodynamically unstable traumatic patients with severe hemoperitoneum for immediate surgery. The detection of intraperitoneal free air or an intraperitoneal gas pocket is very helpful for bedside diagnosis of acute abdomen and trauma patients. The detection of a gas pocket may support doctors to assess, although not in a direct way, if there is an abnormal gas pocket from blunt abdominal trauma or from acute abdomen in (a) pre-hospital settings, (b) initial evaluation in the emergency room, and (c) follow-up after some treatments.
The sonographic appearance of gas pockets is due to total ultrasound reflection (a strong reflector) at the interface of soft tissue and gas pocket (air). This reflection is accompanied by reverberation of ultrasound between the gas pocket and the ultrasound probe. Therefore, sonographic images for gas pockets typically appear as high-amplitude echoes (brightness area in the image) with distal artifactual reverberation echoes referred to as “dirty shadowing”; small reverberation artifacts have a characteristic comet-tail appearance. Small gas pockets may show little or no distal reverberation artifacts with standard abdominal transducers. The optimal probe position for detecting intraperitoneal free air after blunt abdominal trauma is in the right paramedian epigastric area in the longitudinal direction.
Muradali et al. (“A specific sign of pneumoperitoneum on sonography: enhancement of the peritoneal stripe”, AJR, 1999, Vol. 173:1257-1262) studied the signs of pneumoperitoneum from animal models, which were then confirmed in patients who had undergone laparoscopy. This kind of characteristics of gas pockets in the ultrasound images is called Enhancement of the Peritoneal Stripe Sign (EPSS). This EPSS is further confirmed by recent prospective study of six hundred consecutive patients with acute abdominal pain. The EPSS had a sensitivity of 100%, a specificity of 99%, a positive predictive value of 87.5% and a negative predictive value of 100%. Therefore, EPSS is recommended as a reliable and accurate sonographic sign for the diagnosis of pneumoperitoneum through visual observation.
Conventional gas pocket detection based on ultrasound imaging works well if gas pockets are quite large, i.e. large enough to produce enhancement of the peritoneal stripe sign (EPSS). It takes a long time for non-experienced users to identify this kind of sign (EPSS) from ultrasound images. It is really difficult for emergency physicians to identify all gas pockets in an examination time of around 5 minutes for the whole abdomen even if they know the optimal probe position for detecting intraperitoneal free air after blunt abdominal trauma in the right paramedian epigastric area in the longitudinal direction.
In summary, larger gas pockets may appear as bright, highly echogenic stripes or lines with distal reverberation and dirty shadowing artifacts or comet-tail artifacts which may even obscure the underlying abdominal organs. Smaller gas pockets can appear as bright punctuate foci without ring-down artifacts and shadowing within the intestinal lumen, but may not have reverberation within the image. Ultrasound imaging is superior to chest X-rays for diagnosing intraperitoneal free air; quantities as small as 1 ml to 2 ml of intraperitoneal free air can be detected by ultrasound. However, the detection of intraperitoneal free air might be difficult even for an experienced sonographer in emergency situations under difficult patient conditions.
Thus, most of the existing gas pocket detection approaches are based on visual observations of B-mode ultrasound images showing comparisons between normal soft tissue and gas pockets. Such existing approaches are time-consuming and the accuracy is very dependent on the operator's experience.